A bearing for use in, for example, an alternator which is an auxiliary device of an automotive engine, is used under severe conditions such as a high speed rotation, a high temperature and a heavy load. Therefore, a lubricant (grease) to be used for such bearing as described above has been required to maintain a sufficient lubricity for a long period of time without causing problems such as seizure of the bearing under the above-described conditions and, accordingly, various types of greases or grease compositions for rolling bearings have so far been proposed.
In order to comply with such requirement, the present inventors have proposed a grease composition for a rolling bearing comprising a lubricating base oil comprising a poly-α-olefin synthetic oil or a diphenyl ether synthetic oil, a diurea thickener and an organic antimony compound or an organic molybdenum compound as a lubricant which can maintain an excellent lubricity for a long period of time under a high temperature condition (about 170° C. or more) (refer to WO94/03565 pamphlet).
Further, the present inventors have proposed a grease composition for a rolling bearing comprising a lubricating base oil comprising a fluorinated synthetic oil, a polytetrafluoroethylene thickening agent and an organic antimony compound or an organic molybdenum compound, as a lubricant capable of being used under a higher temperature (about 200° C. or more) (refer to JP-A-2000-303088).
On the other hand, in a recent trend in which an importance of corresponding to an environmental problem is increasing, a requirement for reduction of an amount of a heavy metal to be used which is suspected of giving a damage to a global environment or for disuse thereof is increasing. Particularly, antimony (Sb) was designated as a substance which falls in environmental standard articles in the revised environmental standard for water quality in 1993. For this account, although the heavy metal contained in the above-mentioned grease for the rolling bearing is not, today, designated as an object for being restricted by a law, it is well anticipated that usage of the heavy metal is restricted in the future and it becomes necessary for the heavy metal to be replaced by a less hazardous element.